How to cancel visa and take another offer if the employer refuses

I got a job offer from another company and resigned from my current company, giving a notice period of 30 days.

The employer refused to accept my resignation and stated that they will not be cancelling my visa due to COVID-19 issues since they cannot hire anyone else (This is just an excuse they are making).

I wish to join the new company, and I want to know:

1. How can I get my visa cancelled if the employer refuses, without the delays of weeks and months in the courts?

2. How to start working for the new employer in case the current does not cancel the visa, so I don’t lose the job offer?

3. How long does the permit process take?